1. Field of the Invention
This invention relates to electrical devices which comprise at least two electrodes and at least one PTC element.
2. Statement of the Prior Art
It is well known that the resistivity of many conductive materials changes with temperature. The terminology which has been used in the past to describe the different kinds of resistance/temperature variation is variable and often imprecise, but broadly speaking, materials which increase in resistivity are designated PTC (positive temperature coefficient) materials; those that decrease in resistivity are designated NTC (negative temperature coefficient) materials; and those which show no substantial change in resistivity are designated CW (constant wattage) or ZTC (zero temperature coefficient) materials. However, some materials show quite different resistance/temperature characteristics in different temperature ranges; for example a material may show CW behavior at low temperatures and PTC behavior at higher temperatures and/or may show, in a specific temperature range, a very much greater change in the rate at which resistivity changes with temperature than outside that range.
In this specification, the terms "composition exhibiting PTC behavior" and "PTC composition" are used to denote a composition having at least one temperature range (hereinafter called a "critical range") which is within the limits of -100.degree. C. and about 350.degree. C.; at the beginning of which the composition has a resistivity below about 10.sup.5 ohm. cm.; and in which the composition has an R.sub.14 value of at least 2.5 or an R.sub.100 value of at least 10 (and preferably both), and preferably has an R.sub.30 value of at least 6, where R.sub.14 is the ratio of the resistivities at the end and the beginning of a 14.degree. C. range, R.sub.100 is the ratio of the resistivities at the end and the beginning of a 100.degree. C. range, and R.sub.30 is the ratio of the resistivities at the end and the beginning of a 30.degree. C. range. The term "PTC element" is used herein to denote an element composed of a PTC composition as defined above. A plot of the log of the resistance of a PTC element, measured between two electrodes in contact with the element, against temperature, will often, though by no means invariably, show a sharp change in slope over a part of the critical temperature range, and in such cases, the term "switching temperature" (usually abbreviated to T.sub.s) is used herein to denote the temperature at the intersection point of extensions of the substantially straight portions of such a plot which lie either side of the portion showing the sharp change in slope. The PTC composition in such a PTC element is described herein as having "a useful T.sub.s ". The term "anomaly temperature" has also been used in the past to denote the temperature at which a PTC element shows a sharp increase in the rate at which resistance increases with temperature.
PTC compositions and electrical devices, especially heaters, which contain PTC elements, have been described in a number of publications. Reference may be made for example to U.S. Pat. Nos. 2,978,665; 3,243,753; 3,351,882; 3,412,358; 3,413,442; 3,591,526; 3,673,121; 3,793,716; 3,823,217; 3,858,144; 3,861,029; 3,914,363 and 4,017,715; British Pat. No. 1,409,695; Brit. J. Appl. Phys. Series 2, 2 569-576 (1969, Carley Read and Stow); Kautschuk und Gummi II WT, 138-148 (1958, de Meij); Polymer Engineering and Science, Nov. 1973, 13, No. 6, 462-468 (J. Meyer); U.S. Patent Office Defensive Publication No. T 905,001; German Offenlegungschriften Nos. 2,543,314.1, 2,543,338.9, 2,543,346.9, 2,634,931.5, 2,634,932.6, 2,634,999.5, 2,635,000.5, and 2,655,543.1; and German Gebrauchsmuster No. 7,527,288. Reference may also be made to U.S. Patent Application Ser. Nos. 601,424, now abandoned (and the continuation thereof Ser. No. 790,977, now abandoned), 601,549, now abandoned (and the CIP thereof Ser. No. 735,958, now abandoned), 601,550, 601,638, now abandoned, 601,639, 608,660, 638,440, now abandoned (and the CIP thereof Ser. No. 775,882), 732,792, now abandoned, 750,149, now abandoned, 751,095 and 798,154, now abandoned. The disclosure of each of these publications and applications is hereby incorporated by reference.
As discussed in U.S. Application Ser. No. 601,638 and the corresponding German Offenlegungschrift No. 2,543,314.1, current inrush is an important problem which can arise in the use of electrical devices containing PTC elements, especially heaters. Such devices are usually used in a way such that the PTC element is initially at room temperature when current first passes through it, but subsequently operates at an elevated working temperature at which its resistance is substantially higher than at room temperature. As a result, when current is passed through the PTC element, the size of the initial current in the circuit containing the PTC element can be very much greater than it is at a later stage when the device is at its operating temperature. If, as in the case of heaters, a substantial current is required for effective operation at the operating temperature, the size and duration of the initial current can be such that the device itself or other components of the circuit can be permanently damaged, unless precautions are taken to prevent this initial current inrush. Another important problem which can arise in the use of electrical devices containing PTC elements, especially heaters, is the formation of "hotlines" in the PTC element. As discussed in U.S. Ser. No. 601,638 and the corresponding German Offenlegungschrift No. 2,543,314.1, and also in U.S. Ser. No. 608,660 and the corresponding German Gebrauchsmuster No. 7,527,288, if the preferred current path through a relatively thin PTC element is transverse to the thickness of the element, then as the temperature of the element increases, there is a tendency for a part of the element, extending across the thickness of the element, to be heated much more rapidly than the remainder, thus giving rise to a so-called "hot-line". The presence of a hot-line seriously reduces the heat output of a PTC element, because relatively little heat is generated outside the hot-line; in addition the presence of a hot-line renders the heat output non-uniform and can damage the PTC element.
U.S. Ser. No. 601,638 and the corresponding German Offenlegungschrift No. 2,543,314.1 describe inter alia electrical devices which comprise at least two electrodes, at least one first electrically resistive layer and at least one second electrically resistive layer; at least a part of the surface of the first layer being contiguous with at least a part of the surface of the second layer; the first layer exhibiting a positive temperature coefficient of resistance and having an anomaly temperature; the second layer having a substantially constant resistance (as defined in said application and Offenlegungschrift) below the anomaly temperature of the first layer; and the electrodes and the resistive layers being such that, at the higher of (a) the anomaly temperature of the first layer, and (b) the temperature at which the resistance of the first layer exceeds the resistance of the second layer, current flowing between the electrodes predominantly follows the directionally shortest path through the first layer. As described in detail in said application and Offenlegungschrift, in such devices the formation of "hot-lines" is substantially avoided. In addition, the said application and Offenlegungschrift teach that by observing certain restrictions on the positioning of the electrodes and the relative resistivities of the resistive layers in such devices, the problem of current inrush can be substantially reduced. While the invention described in said application and Offenlegungschrift is extremely valuable, the restrictions referred to above mean that it does not provide a solution to the problem of current inrush which is satisfactory in all cases.